Conventionally, in an acoustic content such as music or broadcasting, two-channel content is mainly used. The two-channel content is configured of a left-channel acoustic signal FL which is reproduced from a speaker located at a position diagonally to the left-front of a user and a right-channel acoustic signal FR which is reproduced from a speaker located at a position diagonally to the right-front of the user.
In the 1990s, various 5.1 channel sound formats typified by Dolby Digital System have been proposed, and the 5.1 channel sound contents which comply with such a format are recorded on DVDs and the like and have become widely available as goods. The 5.1 channel sound content is configured of, in addition to the channels FL and FR, a center channel FC which is reproduced from a speaker located at a position directly in front of the user, a left surround channel RL which is reproduced from a speaker located at a position diagonally to the left-rear of the user, a right surround channel RR which is reproduced from a speaker located at a position diagonally to the right-rear of the user, and an acoustic signal of a channel LFE which is reproduced from a speaker exclusively used for low frequency components of approximately 120 Hz or less. By listening to reproduction sound of acoustic signals of respective channels of the six speakers located so as to surround the user, he or she is able to enjoy higher presence.
Furthermore, in recent years, along with digitalization of television broadcasting wave, the 5.1 channel sound content is adopted in some broadcasting. Thus, the user has more opportunities to enjoy the 5.1 channel sound content. Whereas, it is generally difficult to set six speakers in a limited living space, and there has been an increased demand for more easily enjoying the higher presence obtained from the 5.1 channel sound content.
As a technique for satisfying this demand, a technique referred to as Virtual Surround has been widely used. In this technique, a predetermined head acoustic transfer function is previously embedded with an acoustic signal of each of the channels, so as to reproduce the acoustic signal of each of the channels by a headphone, thereby localizing an acoustic image in a direction in which each of the six speakers are disposed. However, this technique has problems in that the user may feel tired when he or she wears a headphone for a long period of time or the user may feel that acoustic images are so close that they are localized in the vicinity of the head of the user. Thus, the technique has not yet widely spread.
In order to solve this problem, proposed has been a technique for realizing a virtual surround technique, using a head acoustic transfer function, which utilizes a head acoustic transfer function by means of two speakers located at positions diagonally to the right-front and left-front of the user (patent document 1, for example). Hereinafter, a conventional acoustic image localization system 10 which realizes the virtual surround technique by using two speakers will be described with reference to FIG. 32. FIG. 32 is a diagram describing a configuration of the conventional acoustic image localization system 10. Note that in an example of FIG. 32, an acoustic signal of 0.1 channel (channel LFE) is not shown and will not be described. Also, FIG. 32 is a diagram as viewed from above the head of a user 3 who is a listener, and the user 3 faces leftward in the diagram.
In FIG. 32, a multi-speaker system 1 outputs acoustic signals of 5 channels to an acoustic image localization system 10. Specifically, the multi-speaker system 1 outputs, as acoustic signals, a left front channel signal FL, a center channel signal FC, a right front channel signal FR, a left surround channel signal RL and a right surround channel signal RR. Under normal circumstances, these acoustic signals are radiated as acoustic waves outputted from the left front speaker FL, the center speaker FC, the right front speaker FR, the left surround speaker RL and the right surround speaker RR, all of which are shown by dashed lines, i.e., from the five speakers disposed so as to surround the user 3.
The acoustic image localization system 10 causes effect imparting sections 111a to 111e to perform a predetermined effect imparting process on the acoustic signals of 5 channels, and also causes adders 112a to 112h to combine the results of the effect imparting processes. Furthermore, the acoustic image localization system 10 causes a crosstalk canceller 113 to perform a crosstalk cancellation process and output the obtained results via the two speakers which are a left speaker 2a and a right speaker 2b. By executing such processes, the acoustic image localization system 10 provides the user with presence effect as if he or she feels that acoustic waves are radiated from the five speakers.
Each of the effect imparting sections 111a to 111e localizes an acoustic image at a position at which each of the five speakers shown in dotted lines are disposed, and adjusts an amplitude frequency characteristic of an inputted acoustic signal so as to impart an acoustic transfer function corresponding to a position of each of the five speakers. Hereinafter, a process executed by the effect imparting section 111a will be described, for example. The effect imparting section 111a localizes an acoustic image at a position of the right surround speaker RR, and adjusts an amplitude frequency characteristic of an inputted acoustic signal so as to impart an acoustic transfer function corresponding to the position of the right surround speaker RR. More specifically, the effect imparting section 111a is designed as a filter for reproducing an acoustic transfer function HL from the position of the right surround speaker RR to a left ear of the user 3 and an acoustic transfer function HR from the position of the right surround speaker RR to a right ear of the user 3. With the effect imparting process executed by the effect imparting section 111a, the effect imparting section 111a outputs an acoustic signal having an amplitude frequency characteristic of the acoustic transfer function HL as a left-ear acoustic signal. Also, the effect imparting section 111a outputs an acoustic signal having an amplitude frequency characteristic of the acoustic transfer function HR as a right-ear acoustic signal.
FIG. 33 shows time-axis responses (impulse responses) of the acoustic transfer functions HL and HR, and amplitude frequency characteristics of the acoustic transfer functions HL and HR. The right surround speaker RR is located at a position 120 degrees diagonally to the right-rear of the user 3. FIG. 33(a) is a diagram showing the time-axis responses of the acoustic transfer functions HL and HR. FIG. 33(b) is a diagram showing the amplitude frequency characteristics of the acoustic transfer functions HL and HR. As is clear from FIG. 33(a), in the speaker located at a position diagonally to the right-rear of the user 3, an acoustic pressure response value of the time-axis response of the acoustic transfer function HR is different from that of the time-axis response of the acoustic transfer function HL. Also, as is clear from FIG. 33(b), in the speaker located at a position diagonally to the right-rear of the user 3, the amplitude frequency characteristic of the acoustic transfer function HR is different from that of the acoustic transfer function HL. Due to these differences, in the prior art, an amplitude frequency characteristic of an acoustic transfer function from a position at which an acoustic image should be localized to each ear has been a significant factor to localize an acoustic image. The conventional acoustic image localization system 10 adopts a control method in which the acoustic transfer functions HL and HR from a position at which an acoustic image should be localized (the right surround speaker RR) to both ears of the user 3 are faithfully reproduced at the positions of both ears. Specifically, the conventional acoustic image localization system 10 causes the effect imparting sections 111a to 111e to perform the effect imparting process and causes the crosstalk canceller 113 to perform the crosstalk cancellation process, thereby faithfully reproducing the acoustic transfer functions HL and HR at the positions of both ears of the user 3.
The effect imparting section 111a is designed by an FIR-type filter using a filter coefficient which is a discrete value of a time-axis response value for each of the right and left ears. Thus, the left-ear acoustic signal outputted from the effect imparting section 111a becomes an acoustic signal having a faithful amplitude frequency characteristic of the acoustic transfer function HL, and the right-ear acoustic signal becomes an acoustic signal having a faithful amplitude frequency characteristic of the acoustic transfer function HR.
It is assumed that the left speaker 2a radiates left-ear reproduction sound reproduced based on the left-ear acoustic signal, and the right speaker 2b radiates right-ear reproduction sound reproduced based on the right-ear acoustic signal. In this case, not only the left-ear reproduction sound radiated from the left speaker 2a but also the right-ear reproduction sound radiated from the right speaker 2b arrive at the left ear of the user 3. Similarly, not only the right-ear reproduction sound radiated from the right speaker 2b but also the left-ear reproduction sound radiated from the left speaker 2a arrive at the right ear of the user 3. As such, reproduction sound is leaked to an ear different from an ear to which the reproduction sound should be conveyed (crosstalk occurs). Due to the crosstalk, it is impossible to obtain an amplitude frequency characteristic of a faithful acoustic transfer function corresponding to a position of the right surround speaker RR at which an acoustic image is localized at each ear of the user 3.
The crosstalk canceller 113 adjusts a phase frequency characteristic of an inputted acoustic signal in order to cancel the crosstalk. Specifically, cancel sound having a phase opposite to the left-ear reproduction sound radiated from the left speaker 2a is radiated from the right speaker 2b at the same time when the reproduction sound is radiated from the left speaker 2a. Similarly, cancel sound having a phase opposite to the right-ear reproduction sound radiated from the right speaker 2b is radiated from the left speaker 2a at the same time when the reproduction sound is radiated from the right speaker 2b. By executing the above process, the crosstalk is cancelled. As a result, the acoustic transfer functions HR and HL from the position of the right surround speaker RR at which an acoustic image should be localized to right and left ears are faithfully reproduced, and therefore the user 3 is able to listen to sound represented by the acoustic transfer function HL shown in FIG. 33 with the left ear, and is also able to listen to sound represented by the acoustic transfer function HR shown in FIG. 33 with the right ear. Thus, the user 3 is able to feel as if sound is radiated from the right surround speaker RR (hereinafter, referred to as acoustic image localization effect).
Note that the aforementioned processes are executed in the similar manner in the effect imparting sections 111b to 111e. As a result, the conventional acoustic image localization system 10 shown in FIG. 32 provides the user 3 with an acoustic image localization effect that he or she can feel as if sound is radiated from the five speakers disposed to surround the user 3.
As described above, in the conventional acoustic image localization system 10, an acoustic transfer function from a position at which an acoustic image should be localized to each ear is faithfully realized by means of the effect imparting processes executed by the effect imparting sections 111a to 111e and the crosstalk cancellation process executed by the crosstalk canceller 113, in order to provide the user 3 with the acoustic image localization effect.
In the conventional acoustic image localization system 10, however, a control parameter of the crosstalk canceller 113 needs to be set based on a listening position of the user 3 which has been previously simulated. Furthermore, in the case where the user 3 moves his or her head and the listening position changes, the phase frequency characteristics represented by the acoustic transfer functions from the left speaker 2a to the left and right ears of the user 3 and from the right speaker 2b to the left and right ears of the user 3 accordingly change. As described above, when a listening position is shifted from a position which has been previously simulated, a phase of the cancel sound is not completely opposite to a phase of the reproduction sound, thereby deteriorating the crosstalk cancellation effect. Furthermore, a wavelength of an acoustic wave is short in a high frequency band. Therefore, in the high frequency band, the range in which a phase of cancel sound is completely opposite to a phase of the reproduction sound is extremely narrow. Thus, the cross cancellation effect is heavily deteriorated.
As shown in FIG. 33, an amplitude level of the amplitude frequency characteristic of the acoustic transfer function HL from the right surround speaker RR to the left ear greatly fluctuates in the high frequency band. The same is also true of an amplitude level of the amplitude frequency characteristic of the acoustic transfer function HR. From this result, it is apparent that the amplitude frequency characteristics in the high frequency band exerts a great influence upon the acoustic image localization effect. Therefore, in the conventional acoustic image localization system 10, even if a listening position slightly changes, the crosstalk cancellation effect heavily deteriorates in the high frequency band. Thus, the acoustic transfer function from a position at which an acoustic image should be localized to each ear of the user 3 cannot be faithfully reproduced. What is worse, the acoustic image localization effect cannot be significantly obtained.
In practical use, the user 3 never always keeps the same posture when listening to sound, and the user 3 hardly listens to sound at a listening position which has been simulated when the crosstalk canceller 113 is designed. Thus, in practical, a listening position which has been previously simulated hardly coincides with a position of each ear of the user 3, whereby the acoustic image localization effect is hardly obtained.
As described above, in the conventional acoustic image localization system 10, since the crosstalk canceller 113 executes the crosstalk cancellation process, the listening position range in which the acoustic image localization effect can be obtained is extremely narrow. Furthermore, in practical, the acoustic image localization effect is hardly obtained.
For solving these problems, an acoustic reproduction system capable of suppressing deterioration of the crosstalk cancellation effect in the high frequency band and capable of producing the acoustic image localization effect within a wide listing range (patent document 2, for example). Hereinafter, a conventional acoustic reproduction system capable of producing the acoustic image localization effect within a wide listening range will be described with reference to FIG. 34. The acoustic image reproduction system includes an acoustic localization system 11, the left speaker 2a, the right speaker 2b, and a cabinet 12. The acoustic localization system 11 is connected to the left speaker 2a and the right speaker 2b. Note that the left speaker 2a, the right speaker 2b and the user 3 shown in FIG. 34 are the same as those shown in FIG. 32, and the above components are denoted by the same reference numerals. FIG. 34 is a diagram as viewed from above of the head of the user 3 and the user 3 faces upward in the diagram.
In FIG. 34, the left speaker 2a and the right speaker 2b are attached to the cabinet 12 and are disposed so as to be adjacent to each other. The left speaker 2a and the right speaker 2b are positioned such that a forward angle θ from the position of the user 3 is within a range from 6 to 20 degrees.
The acoustic localization system 11 includes digital filters 121a to 121d, and adders 122a and 122b. The acoustic localization system 11 processes a plurality of acoustic signals u1 and u2, and outputs output signals v1 and v2 for running the left speaker 2a and the right speaker 2b. Note that the acoustic signals u1 and u2 represent normal stereo signals (acoustic signals of channels FL and FR). The digital filters 121a to 121d are designed so as to perform the crosstalk cancellation process. More specifically, the digital filters 121a to 121d are designed so as to have a processing characteristic for causing an acoustic transfer function of a position of each ear of the user 3 to coincide with a head acoustic transfer function which localizes the acoustic signal u1 or u2 in a predetermined direction. The detailed design method has been disclosed in European Patent Publication No. 0434691, Patent Specification No. WO 94/01981 and the like.
In the acoustic reproduction system shown in FIG. 34, the left speaker 2a and the right speaker 2b are disposed so as to adjacent to each other, thereby suppressing the deterioration of cancellation effect in the high frequency band and thus providing the user with the acoustic image localization effect within the wide listing range. Hereinafter, the reasons therefor will be described with reference to FIG. 35. FIG. 35 is a diagram schematically showing wavefronts of reproduction sound and cancel sound.
In FIG. 35, a plurality of arc-shaped dotted lines extending forward from the right speaker 2b show wavefronts having phases of 180 degrees with respect to wavefronts of reproduction sound arrived from the right speaker 2b to the left ear of the user 3. Also, a plurality of arc-shaped solid lines extending forward from the left speaker 2a show wavefronts having phases of 0 degrees with respect to wavefronts of cancel sound reproduced by the left speaker 2a. In areas in which the dotted lines of the right speaker 2b overlap the solid lines of the left speaker 2a, a phase of the cancel sound reproduced by the left speaker 2a is opposite to that of the reproduction sound arrived from the right speaker 2b to the user 3. Note that in FIG. 35, the left speaker 2a and the right speaker 2b are disposed so as to be adjacent to each other. Therefore, as shown in FIG. 35, the ark-shaped dotted lines of the right speaker 2b and the ark-shaped solid lines of the left speaker 2a greatly overlap with each other. That is, a range in which a phase of cancel sound from the left speaker 2a is opposite to that of reproduction sound from the right speaker 2b becomes wider. As such, in the acoustic reproduction system shown in FIG. 34, the left speaker 2a and the right speaker 2b are disposed so as to be adjacent to each other, thereby suppressing the deterioration of crosstalk cancellation effect in the high frequency band and thus providing the acoustic image localization effect within the wide listing range.
[Patent document 1] Japanese Laid-Open Patent Publication No. 9-200897
[Patent document 2] Japanese Unexamined Patent Publication No. 2000-506691